Large radial movement compliant seal

ABSTRACT

A compliant seal assembly having a seal pack carrier having an upstream face and a downstream face that defines a slot, and a seal pack having an upstream face and a downstream face slideably mounted within the slot. The seal pack carrier and the seal pack contact each other and form a high-pressure plenum and a low-pressure plenum. The compliant seal assembly also includes a first passage with a first and second opening and a second opening with a first and second opening. The first opening of the first passage is located at the upstream face of the seal pack carrier, and the second opening of the first passage communicates with the high pressure plenum. The first opening of the second passage is located at the downstream face of the seal pack carrier, and the second opening of the second passage communicates with the low-pressure plenum.

The present invention relates to compliant seals used to provide sealingbetween fluids at different pressures. In particular, the presentinvention relates to compliant seals used to provide sealing between anupstream region of high pressure gas and a downstream region ofrelatively low pressure gas, especially where the regions are bound byrelatively rotatable components such as the shaft and casing of a gasturbine engine.

Compliant seals in gas turbine engines utilise a continuous annularsealing face, which seals against, typically, a rotating shaft. Thissealing surface is formed from a number of flexible elements, whichproject radially inwards from a rigid annular carrier, to which they arefixed. The element's flexibility allows the sealing face, formed by freefloating radially inner ends of the elements, to be compliant, and soaccommodate movement of the shaft relative to the annular carrier, whichis typically mounted to an engine casing. Such movement arises fromeccentric movement of the shaft relative to the casing, or unequalgrowth of the shaft relative to the casing.

Brush seals and leaf seals are both examples of such compliant seals.Brush seals utilise an annular array of bristles, many layers thick, andare well known in the art. Leaf seals utilise an annular array ofradially inward pointing planar ‘leaves’, typically a single layerthick. These leaves comprise plates, which are very thin in section, sothat they can flex radially, but are axially stiff, to avoid the problemof pressure stiffening associated with brush seals.

U.S. Pat. No. 5,474,305 describes a seal assembly suitable for low speedoperation in which the bristle pack is slideably located in a radialchannel such that it floats radially. This allows the clearance betweenbacking plate and sealing surface to be reduced, as the brush pack isitself able to move to maintain sufficient clearance between the shaftand backing plate. However, in operation, the pressure differentialacross the brush pack pushes it against the downstream face of theradial channel. This causes wear between the brush pack and rear face ofthe channel, and locks the brush pack, increasing the clearance gaprequired.

The present invention addresses the limitations of the prior art citedabove.

According to the present invention, a compliant seal for sealing betweenan upstream region and a downstream region comprises, a seal packcarrier defining a slot, the seal pack carrier having an upstream faceand a downstream face, and a seal pack slideably mounted within theslot, the seal pack having an upstream face and a downstream face,wherein the seal pack carrier and seal pack define a high-pressureplenum which abuts the downstream side of the seal pack, and which canbe pressurised to apply a force to the seal pack in the upstreamdirection, and the seal pack carrier and seal pack further define alow-pressure plenum which abuts the upstream side of the seal pack, andwhich can be vented to increase the effect of the force generated by thehigh-pressure plenum.

Preferably, high-pressure flow communication means are provided betweenan external surface of the seal pack carrier and the high-pressureplenum for supply of a pressurised fluid to the plenum. Thehigh-pressure flow communication means may be provided between theupstream face of the seal and the high-pressure plenum.

Preferably, low-pressure flow communication means are provided betweenan external surface of the seal pack carrier and the low-pressure plenumfor venting of a pressurised fluid from the low-pressure plenum. Thelow-pressure flow communication means may be provided between thedownstream face of the seal and the low-pressure plenum.

Preferably, the seal pack further comprises a compliant annulus and abacking plate, spaced apart to define a seal pack plenum, and secondaryflow communication means between the seal pack plenum and thehigh-pressure plenum.

Preferably, discrete sealing means are provided between the seal packand seal pack carrier.

Preferably, the seal is provided with anti-rotation means to preventrelative rotation between the seal pack and seal pack carrier.

Preferably, biasing means are provided between the seal pack and sealpack carrier, which urge the seal pack against the seal pack carrier.

According to a further aspect of the present invention, there isprovided a gas turbine engine which, during operation of the engine,provides a first upstream region with a fluid at a higher pressure thana fluid in a second downstream region, wherein the upstream region isseparated from the downstream region by a seal, the seal comprising aseal pack carrier defining a slot, the seal pack carrier having anupstream face and a downstream face, and a seal pack slideably mountedwithin the slot, the seal pack having an upstream face and a downstreamface, wherein the seal pack carrier and seal pack define a high pressureplenum which abuts the downstream side of the seal pack, and the sealpack carrier and seal pack further define a low-pressure plenum whichabuts the upstream side of the seal pack, wherein high-pressure flowcommunication means are provided between the high-pressure plenum and aregion of the gas turbine engine which, during operation of the gasturbine engine, contains fluid at higher pressure than fluid in thedownstream region adjacent the seal such that a force is applied to theseal pack in the upstream direction, and low-pressure flow communicationmeans are provided between the low-pressure plenum and the downstreamregion, so that the low-pressure plenum is vented to the downstreamregion.

Preferably, the high-pressure flow communication means are providedbetween the upstream region adjacent the seal and the high-pressureplenum. The high-pressure flow communication means may be providedbetween a separate high-pressure region and the high-pressure plenum.

The invention will now be described in more detail with reference to theaccompanying drawings in which;

FIG. 1 illustrates a cross-section through a compliant seal according toa first embodiment of the present invention;

FIG. 2 shows a view, on section X-X, of the seal pack of the compliantseal shown in FIG. 1;

FIG. 3 shows a view, on section Y-Y, of the seal pack carrier of theembodiment of FIG. 1;

FIG. 4 shows a cross-section through a compliant seal according to asecond embodiment of the present invention;

FIG. 5 shows a view, on section Z-Z, of the seal pack of the compliantseal shown in FIG. 4;

FIG. 6 shows a cross-section through a compliant seal according to afurther embodiment of the present invention;

FIG. 7 shows a cross-section through a compliant seal according to afurther embodiment of the present invention;

FIG. 8 shows, in more detail, a first embodiment of the sealingarrangement used on the compliant seal of FIG. 7;

FIG. 9 shows, in more detail, a second embodiment of the sealingarrangement used on the compliant seal of FIG. 7;

FIG. 10 shows a cross-section through a compliant seal according to afurther embodiment of the present invention;

FIG. 11 shows a cross-section through compliant seal according to afurther embodiment of the present invention;

FIG. 12 shows a cross-section through compliant seal according to afurther embodiment of the present invention;

FIG. 13 shows a cross-section through compliant seal according to afurther embodiment of the present invention; and

FIG. 14 shows a cross-section through compliant seal according to afurther embodiment of the present invention.

FIG. 1 shows a cross-section through part of a gas turbine engine, inwhich a rotatable shaft 2, with axis 3, is mounted, within a static,concentric, casing 4. The annular gap between shaft 2 and casing 4 isclosed by a compliant seal 6, which seals a first, upstream region 8from a second, downstream region 10. During operation of the gasturbine, air in the first region 8 is pressurised to a higher pressurethan air in the low-pressure region 10. The brush seal 6 isolates theregions 8,10 from one another.

The brush seal 6 comprises a seal pack 12 slideably mounted within anannular seal pack carrier 14.

The seal pack 12 comprises a compliant annulus 13, which comprises adense annular array of bristles 16, known as a bristle pack 17, boundabout its external circumference by a retaining member 18. The seal pack12 further comprises an upstream annular cover plate 20, which forms theupstream surface 21 of the seal pack 12, and a downstream annularbacking plate 22, which forms the downstream surface 23 of the seal pack12.

Turning to FIG. 2, which shows a plan view of the brush pack 17, alongthe section X-X of FIG. 1, the bristles 16 are arranged to point inwardsfrom this retaining member 18 to form a sealing face 26 at theirinternal diameter. They are inclined at a lay angle to the radial sothat a length of each bristle, at its radially inner end, lays adjacentthe surface of the shaft 2.

The cover plate 20 is spaced apart from the bristle pack 17. It extendsradially inboards to a diameter 28, such that annular gap A is definedbetween the cover plate 20 and the sealing face 26, sized to accommodatethe maximum likely deflection of the sealing face 26 through radialgrowth of the shaft 2 relative to the seal during engine operation.

The backing plate 22 extends radially inwards from the retaining member18 to an internal diameter 32 such that annular gap B, similar in sizeto annular gap A, is defined between the plate 22 and the sealing face26. The plate 22 supports the downstream face of the bristle pack 17.

The seal pack carrier 14 comprises an annular retaining wall 40, firstradial wall 42, and second radial wall 44. The retaining wall 40 isattached to the engine casing 4, and has an internal diameter 45 largerthan the seal pack 12 external diameter so that an annular clearance Cis formed between the two. This clearance C governs the radial movementof the seal pack 12. It is sized to accommodate the maximum eccentricexcursion between shaft 2 and engine casing 4 during engine operation,and the maximum radial growth of the shaft 2 relative to the carrier 14.

The radial walls 42,44 project radially inwards from, respectively, theupstream and downstream ends of the retaining wall 40, in spaced apartarrangement, to define an annular slot 46, open radially inwards. Theslot's upstream surface 48 is formed by the downstream surface of thefirst radial wall 42, which extends radially inwards to an internaldiameter 50. The upstream surface of the same radial wall 42 forms theexternal, upstream face 51 of the carrier 14.

The slot's downstream surface 52 is formed by the upstream surface ofsecond radial wall 44, which extends radially inwards to an internaldiameter 54, which defines an annular clearance D about the engine shaft2, which is similar in size to annular clearance C. The downstreamsurface 55 of the wall 44 defines the downstream face of the carrier 14.

An annular recess 56 is formed in the upstream slot face 48, bound atits outboard circumference by upstream bearing surface 58, and at itsinner radius by an upstream annular lip 60.

An annular recess 64 is formed in the downstream slot face 52, bound atits outboard circumference by downstream bearing surface 66, and at itsinboard circumference by a downstream annular sealing lip 68.

The seal pack 12 is slideably mounted in the carrier slot 46, the coverplate 20 in sliding contact with the upstream slot surface 48, and thebacking plate 22 in sliding contact with the downstream slot surface 52.

The upstream bearing surface 58 and annular lip 60 seal against theupstream seal pack face 21 to define an annular plenum 62, henceforththe low-pressure plenum 62, which abuts the upstream side of the sealpack 12.

The downstream bearing surface 66 and annular lip 68 seal against thedownstream seal pack face 23 to define an annular plenum 70, henceforththe high-pressure plenum 70, which abuts the downstream side of the sealpack 12.

The upstream carrier wall 42 and downstream carrier wall 44 are sized sothat annular clearance gap D, similar to annular gap C, about the engineshaft, can accommodate the maximum eccentric excursion between shaft 2and engine casing 4 during engine operation, and the maximum radialgrowth of the shaft 2 relative to the carrier 14.

First through passages 72 are formed in the carrier 14 between its frontface 51 and the high-pressure plenum 70. These extend axially throughthe retaining means 40, from the upstream face 51 to a point outboard ofthe high-pressure plenum 70, and turn through 90° to travel radiallyinwards through the downstream carrier wall 44 to the plenum. Thepassages provide high-pressure flow communication means 74 between theupstream region 8 and the high-pressure plenum 70.

Second through passages 76 are formed in the carrier 14 between itsdownstream face 55 and the low-pressure plenum 62. These are shown indashed outline as they lie out of the plane of the first throughpassages 72. The passages 76 extend axially through the retaining means40, from the downstream face 55 to a point outboard of the low-pressureplenum 62, and turn through 90° to travel radially inwards through theupstream carrier wall 42 to the plenum. The passages 76 providelow-pressure flow communication means 78 between the downstream region10 and the low-pressure plenum 62. The arrangement of the high-pressureand low-pressure flow communication means 74,78 will be betterunderstood if reference is now made to FIG. 3.

FIG. 3 shows a section through the carrier retaining wall 40 on the lineY-Y. As can be seen the high-pressure communication means 74 andlow-pressure flow communication means 78 lie within the carrierretaining wall, adjacent to one another about the annulus of the carrier14, separated from one another by an impervious thickness of retainingmeans, in interdigitated arrangement.

During operation of the gas turbine engine, the shaft 2 rotates at highspeed relative to the casing 4. At the same time, air in the region 8upstream of the brush seal 6 is pressurised by a compressor (not shown)to a higher pressure than the downstream region 10. The pressuredifferential across the seal pack applies a load to the seal pack 12, inthe downstream direction, normal to the direction in which the seal packis arranged to slide within the carrier. Unless countered, this loadlocks the seal pack 12 against the carrier 14 and prevents radialmovement of the pack 12.

The high-pressure plenum 70, located on the downstream side of thebristle pack 17, is pressurised to substantially the same pressure asthis upstream region 8 via the high-pressure flow communication means74. The pressurised plenum 70 then applies a force to the seal pack 12in the upstream direction, normal to the direction in which the sealpack 12 is arranged to slide within the seal pack carrier 14. The forcecounters the load applied by the pressure differential across the sealpack 12.

The low-pressure plenum 62 is vented, via the low-pressure flowcommunication means 78 to the downstream region 10. This maintains thelow-pressure plenum 62 at about the same pressure as the downstreamregion 10. Pressure at the upstream slot face 48, absent thelow-pressure plenum 62, would tend to the pressure in the high-pressureregion 8 adjacent the seal 6. Hence, incorporation of the low-pressureplenum 62, increases the force applied by the high-pressure plenum 70 byaugmenting the ‘favourable’ pressure differential across the seal pack12.

The size of the force applied to the seal pack 12 in the upstreamdirection is governed by the pressure of the air in the plenums 62,70,which can be adjusted via the geometry of the communication means 74,78,and by the surface area of the seal pack 12, which abuts the plenums62,70. Hence the force is maximised by increasing the area of the sealpack downstream surface 23 which abuts the high-pressure plenum 70, byincreasing the area of the seal pack upstream surface 21 which abuts thelow-pressure plenum 62, and by optimising the flow communication means74,78 to reduce pressure loss eg by avoiding sharp corners andmaximising flow area. Conversely, where a reduced counter-force isrequired, the areas can be reduced, or throttling means introduced intothe flow communication means 74,78.

Where it is possible to generate an upstream force the same size as thedownstream force applied to the upstream face of the seal pack, a smallimbalance is preferably maintained, such that a light damping load isapplied to the seal pack carrier in the downstream direction to bias itagainst the downstream surface 52 of the seal pack carrier 14. Thisallows the seal pack 12 to move with a minimum of static friction whileproviding damping means to suppress resonance of the seal pack 12 withinthe seal pack carrier 14.

The present brush seal accommodates two distinct forms of relativemovement between the shaft 2 and engine casing 4. Concentric relativemovement between the two is accommodated by deflection of the bristlepack 17, as in a conventional brush seal. Eccentric relative movementbetween the two is accommodated first by deflection of the bristle pack17 until the load applied overcomes the static friction between the sealpack 12 and carrier 14, and then by movement of the seal pack within thecarrier. This provides a much greater range of accommodation ofeccentric movement than has previously been possible. Previous brushseals have used a sliding seal pack to accommodate a certain fixedeccentricity between the first and second components 2,4 but, duringengine operation, the large pressure differential across the slidingseal pack has prevented sliding of the seal pack. Hence such seals havebeen able to accommodate eccentricities arising from engine build, butnot eccentricities arising during engine operation, as are generated by,amongst others, gyroscopic forces on the rotating shaft duringmaneuvers, by large airframe decelerations, such as aircraft landing,and by uneven thermal growth.

The present invention has the advantage that the seal pack 12 as a wholecan move to accommodate excursions by the rotating shaft 2 from the‘perfect’ axis of rotation, located concentric with the casing 4, inparticular during engine operation. As a result, the clearance gap Bbetween backing plate 22 and sealing face 26 is reduced compared with aconventional brush seal or the pressure-balanced brush seal of the priorart. This reduction in the clearance gap B reduces the axial load on thebristle pack 17, allowing bristles 16 to move freely, and hence wear ofthe bristles is reduced. The primary leakage through the seal is alsoreduced as the unsupported annulus of bristles 16 adjacent the sealingface is minimised, which ensures the bristle pack 17 is given themaximum support possible by the backing plate 22. The reduced clearanceB also reduces bristle stress and so maximises the pressure differencethat the seal 6 can maintain.

An alternative version of the present invention is shown in FIGS. 4 and5. FIG. 4 shows a cross section through a compliant seal 70 according tothe present invention, in which the compliant annulus 13 is provided bya leaf seal rather than a brush seal as per the compliant seal 6 ofFIGS. 1, 2 and 3. The seal 80 is otherwise substantially the same as theprevious seal 6, and like items carry like reference numerals.

In the compliant seal 80 of FIGS. 4 and 5, the compliant annulus 13comprises an annular single layer of planar plates 82, called the leafpack 84, in place of the bristle pack 17 of the seal 6 of FIGS. 1,2 and3. The leaf pack 84 will be better understood if reference is made toFIG. 5, which shows a view on cross section Z-Z.

The plates 82 project inward from the retaining member 18, at a layangle to the radial. They are arranged end-on to this lay angle so thatthey are thin in the direction of required compliance and can bend toaccommodate relative movement between the shaft 2 and the casing 4, butare long in the axial direction to resist axial deflection. In the caseof a leaf seal 80, the amount of relative movement between shaft 2 andcasing 4 that can be accommodated by the flexibility of the plates 82 isgreatly enhanced by allowing the leaf pack 84 to move radially relativeto the seal pack carrier.

FIG. 6 shows a further embodiment of a seal 90 according to the presentinvention in which the backing plate 22 of the seal pack 12 is adaptedto provided a plenum on the downstream side of the bristle pack 17,henceforth the seal pack plenum 92. The seal 90 of FIG. 6 is otherwisesubstantially similar to the seal 6 of FIGS. 1,2 and 3, and like itemscarry like reference numbers.

The backing plate 22 of the seal 90 is spaced apart from the bristlepack 17, except at its inner diameter 32 where a continuous annular lip94 engages the bristle pack 17 to support the inner ends of the bristles16 adjacent the sealing face 26. Hence, the seal pack plenum 92 isformed between the backing plate 22 and bristle pack 17 immediatelydownstream of the bristle pack 17. Secondary flow communication meansare provided between this plenum 92 and the high-pressure plenum 70 viaan array of through holes 96 in the backing plate 22. The holes 96 arearranged so that they always lie within the high-pressure plenum 70, andare not occluded over the range of movement of the seal pack 12, or indirect communication with the downstream region 10.

During operation of the seal 90, pressurised air supplied to pressurisethe high-pressure plenum 70 also pressurises the rear face of thebristle pack 17 via the through holes 96 located in the backing plate22. This prevents the compaction of the bristles 16 against the backingplate 22 by balancing pressures on either side of the pack 17 andensures that the full length of the bristle 16 can flex to accommodatemovement at the sealing face 26. This prevents localised bending and theassociated, accelerated, rates of wear of the bristles 16.

It will be understood that the seal pack plenum 92 is equally wellsuited for use with a seal 80 as shown in FIGS. 4 and 5, which utilisesa leaf pack 84 instead of the seal pack 17 of the present invention. Insuch a seal 80, the seal pack plenum 92 is used to control the flowfield through the leaf pack 84.

FIG. 7 shows a seal 100 according to a further embodiment of the presentinvention. The seal 100 is similar to the seal 90 of FIG. 6, and assuch, like parts carry the same reference numerals, however, it isprovided with discrete sealing means between the seal pack 12 andcarrier 14. Because the sliding arrangement of the seal pack 12 and sealpack carrier 14 introduces a potential leakage path between the highpressure and low pressure regions 8,10 it is important that the sealinginterfaces between the seal pack 12 and the seal pack carrier 14 seal atleast as efficiently as the main leakage path, the sealing face 26. Thisis possible using very accurately machined surfaces, but in a preferredembodiment of the present invention, secondary sealing means areprovided between the seal pack 12 and the seal pack carrier 14. Asbefore, the seal pack 12 is slideably mounted within the seal packcarrier 14 however, small annular slots are provided in both slot faces48,52. A first annular slot 102 is formed in the upstream sealing lip60, and an annular sealing element 104 is located within this slot 102,to provide a first discrete seal between the seal pack 12 and seal packcarrier 14, between the upstream slot surface 48 and upstream face 21 ofseal pack. A second annular slot 106 is provided in the downstreamannular sealing lip 68, and a similar annular sealing element 108 islocated therein to provide a second discrete seal between the seal pack12 and seal pack carrier 14, between the downstream slot surface 52 andthe downstream face of the seal pack 23. A third annular slot 110 isprovided in the seal pack backing plate annular lip 94, to provide athird discrete sealing means between the brush pack 17 and the backingplate 22.

FIG. 8 shows one of the annular slots 102 in more detail, and inparticular, a first embodiment of the annular sealing element 104located therein. According to this first embodiment, a circular pistonring seal 114 is provided in the slot, which is substantially the samethickness as the width of the annular slot 102,106. The piston ring issprung outwards from the slot so that it engages the forward surface ofthe seal pack 12 throughout its range of movement, accommodating axialdiscrepancies therebetween. This provides a seal between the ring 114and the seal pack 12, and between the ring 114 and the sides of the slot102 and hence the seal carrier 14. Such seals are well known in the artand will not be described further herein.

FIG. 9 shows in more detail one of the annular slots 102 and a secondembodiment of the annular sealing element 104 located therein. Accordingto the second embodiment, a tubular brush seal 116 is provided in theslot 102 comprising a carrier ring 118, which holds an annular array ofbristle 120 which project axially forward therefrom. The bristles arefree at their axially forward end 122 and spring against the seal pack12 to provide a seal between the seal pack 12 and carrier 14, and toaccommodate axial discrepancies therebetween.

Either of the embodiments of the annular sealing element shown is ableto provide a seal at least as good as that between the shaft 2 andbristle pack 17 because the motion between the seal pack 12 and carrier14, and between the backing plate 22 and bristle pack 17 is much slowerthan the speed of rotation of the shaft 2 relative to the bristle packsealing face 26.

It will be understood that discrete seals as described above areapplicable to all seals according to the present invention. Othersealing means may be utilised, such as O-rings where operatingconditions permit. The discrete sealing means may be used selectively,for example, the third discrete sealing means on the seal pack may beomitted where the seal pack plenum is omitted.

Turning now to FIG. 10, a seal 120 according to a further embodiment ofthe present invention is shown. The seal 120 is substantially the sameas the seal 90 of FIG. 6, and like features have the same referencenumbers, however, the seal pack 12 is provided with anti-rotation meansto ensure that it does not rotate in sympathy with the rotating shaft 2.

The seal 120 has an anti-rotation feature, which comprises a pin 122,which is mounted in the upstream wall of the seal pack carrier 12. Thepin 122 projects axially forwards into a cooperating blind feature 124formed in the downstream face of the seal pack carrier 14. In thepresent embodiment, a single pin 122 is used which extends into a radialslot 126. The slot is approximately twice the radial length as theradial gap C between seal pack 12 and seal pack carrier 14, and the samewidth as the pin 122. Hence, the brush pack can slide radially along theslot 126, and pivot about the pin to accommodate the full range ofmovement of the seal pack 12 within the carrier 14. This arrangementprevents rotational movement of the seal pack 12 by the interaction ofthe pin 122 and hole 124, as the pin 122 is rotated into contact withthe sidewall of the slot 126.

It will be understood that more than one pin 122 and cooperating blindhole 124 can be used where the rotational load applied to the seal pack12 by the rotating shaft 2 is too great for a single pin arrangement. Insuch a case, the second hole is a circular cavity of radius similar toannular gap C to prevent any cross-key effects that would serve to lockthe brush pack in place. In a further alternative arrangement, a pair ofsuch circular cavities of radius similar to annular gap C may be used.

Similarly, it will be understood that other anti-rotation means could beused such as a pin located in the seal pack carrier 14 arranged toengage a recess in the seal pack 12. It is important though that theanti-rotation means should not hinder the freedom of the seal pack 12 tomove within the carrier 14.

FIG. 11 shows a cross-section through a brush seal 130 according to adifferent embodiment of the present invention. The seal 130 issubstantially the seal of FIG. 6, and like items carry like references,except that the high-pressure flow communication means 74 does notsupply air from the upstream region 8 of the brush seal. Instead,passages 132 are provided between the plenum 70 and the external face ofthe external wall of the carrier 14. These passages 132 and cooperatingholes 134 formed in the casing 4 provide the high-pressure flowcommunication means 74 between the plenum 70 and a separate highpressure region 138 located outboard of the casing, which is suppliedwith pressurised air during engine operation by a compressor (notshown). Preferably, the high-pressure region 138 operates at a higherpressure than the high-pressure region 8 upstream of the seal 130. Thisembodiment is useful where the area of the seal pack 12 exposed to thehigh-pressure plenum 70 is insufficient, when supplied with air from theupstream region 8, to generate a useful opposing force in the upstreamdirection. In such a situation, a separate source of air, at higherpressure than the upstream region 8, is used to increase the forceavailable from a given plenum size. With such an arrangement, it ispossible to substantially balance the loads across the seal pack 12 sothat seal pack 12 is pressure balanced, that is to say that the airpressure loads on the front and rear face of the seal pack 12 aresubstantially the same. Preferably, in such a design, a slight imbalancebetween loads is maintained so that the seal pack 12 engages the sealcarrier 14 such that friction between the two will dampen resonanceduring engine operation.

FIG. 12 shows a seal 140 according to a further embodiment of thepresent invention. The seal 140 is similar to the seal 90 of FIG. 6, andas such, like parts carry the same reference numerals, however, the seal140 is provided with resilient means 142, which serve to bias the sealpack 12 against the carrier 14 when there is no pressure load across theseal 140, such as the case when the engine is not operating.

The biasing means comprises a spring 144, which is located in acooperating recess 146 in the upstream face of the seal pack 12. Thespring 144 is loaded in compression so that it pushes against theupstream face 48 of the carrier slot. This urges the seal pack 12 in thedownstream direction, normal to the sliding direction of the seal packwithin the carrier, ie the direction that the pressure differentialpushes the seal pack 12 during engine operation. This applies a smallclamping load between the seal pack 12 and seal carrier 14 when there isno pressure differential across the seal 140 so that the seal pack 12does not drop to the bottom of radial gap C when the engine is notoperating. This avoids excessive wear of the sealing face 26 at top deadcentre, which would otherwise have to support the entire weight of theseal pack 12, during engine start up.

FIG. 13 shows a seal 150, which is another arrangement of the seal 140of FIG. 12, and, again, like items carry like reference numerals. In theseal 150 of FIG. 13, biasing means 152 are provided which comprise anannular ‘cockle spring’ 154 which lies within the annular recess 64 ofthe seal pack carrier, on the downstream side of the seal pack 12. Thiscockle spring 154 applies an upstream load to the seal pack 12 normal tothe sliding direction of the seal pack 12, so that it is urged againstthe upstream face 48 of the carrier slot 46. It will be understood thatthe cockle spring 154 could be similarly arranged in the low-pressureplenum, however, an advantage of the arrangement shown is that the sealpack 12 is urged in the upstream direction, hence the biasing means 152augments the force generated by the high-pressure plenum 70 duringengine operation.

FIG. 14 shows a seal 160, which is another embodiment of the seal 90 ofFIG. 6, in which like items carry like reference numerals. The radiallyinner faces 28,32 of the cover plate 20 and backing plate 22 areprovided with an abradable coating 162. It will be understood that theinternal diameter of each wall when uncoated is larger than the requireclearance gap, A and B respectively, so that the coated walls are thecorrect internal diameter. The coating 162 is significantly softer thanthe material of the shaft 2 so that the coating 162 wears in preferenceto the shaft 2 upon contact. This allows the clearance gaps A and B tobe run closer as the inner surfaces 28,32 can adapt, via localisedrubbing and wear, to accommodate any eccentricities between shaft 2 andcasing 4. It also protects the shaft 2 in the event of contact with theseal pack 12.

The coating 162 can be applied in similar fashion to the inner diameter54 of the carrier walls 42,44.

Although the embodiments described hereinbefore describe the use ofsingle continuous annular chambers to form plenums 62,70,92 it will beunderstood that such a construction is not intended to limit the scopeof the present invention. For example, a number of discrete arcuatechambers may be used instead, the radial spokes between them providingan increased downstream bearing surface. Similarly, a plurality ofconcentric annular chambers may be used.

1. A compliant seal assembly comprising: a seal pack carrier defining aslot, the seal pack carrier having an upstream face and a downstreamface, a seal pack slideably mounted within the slot, the seal packhaving an upstream face and a downstream face, wherein the seal packcarrier and the seal pack contact each other to form a high-pressureplenum which abuts the downstream face of the seal pack, and which canbe pressurized to apply a force to the seal pack in an upstreamdirection, and the seal pack carrier and the seal pack further define alow-pressure plenum which abuts the upstream face of the seal pack, andwhich can be vented to increase an effect of force generated by thehigh-pressure plenum, and wherein the compliant seal assembly furthercomprises: a first passage with a length between a first opening and asecond opening, wherein the first opening of the first passage islocated at an external surface of the upstream face of the seal packcarrier, and the second opening of the first passage communicates withthe high-pressure plenum, and a second passage with a length between afirst opening and a second opening, wherein the first opening of thesecond passage is located at an external surface of the downstream faceof the seal pack carrier, and the second opening of the second passagecommunicates with the low-pressure plenum.
 2. A compliant seal assemblyas claimed in claim 1, wherein high-pressure is provided to thehigh-pressure plenum through the first passage for supply of apressurized fluid to the high-pressure plenum.
 3. A compliant sealassembly as claimed in claim 1, wherein low-pressure is provided byventing of a pressurized fluid from the low-pressure plenum through thesecond passage.
 4. A compliant seal assembly as claimed in claim 1,wherein the seal pack further comprises a compliant annulus and abacking plate, spaced apart to define a seal pack plenum, and flowcommunication means between the seal pack plenum and the high-pressureplenum.
 5. A compliant seal assembly as claimed in claim 1, whereindiscrete sealing means are provided between the seal pack and the sealpack carrier.
 6. A compliant seal assembly as claimed in claim 1,wherein the compliant seal is provided with anti-rotation means toprevent relative rotation between the seal pack and the seal packcarrier.
 7. A compliant seal assembly as claimed in claim 1, whereinbiasing means are provided between the seal pack and the seal packcarrier, which urge the seal pack against the seal pack carrier.
 8. Agas turbine engine which, during operation of the engine, provides afirst upstream region with a fluid at a higher pressure than a fluid ina second downstream region, wherein the upstream region is separatedfrom the downstream region by a compliant seal assembly, the compliantseal assembly comprising a seal pack carrier defining a slot, the sealpack carrier having an upstream face and a downstream face, a seal packslideably mounted within the slot, the seal pack having an upstream faceand a downstream face, wherein the seal pack carrier and the seal packcontact each other to form a high-pressure plenum which abuts thedownstream face of the seal pack, and the seal pack carrier and the sealpack further define a low-pressure plenum which abuts the upstream faceof the seal pack, and wherein the compliant seal assembly furthercomprises: a first passage with a length between a first opening and asecond opening, wherein the first opening of the first passage islocated at an external surface of the upstream face of the seal packcarrier, and the second opening of the first passage communicates withthe high-pressure plenum, and a second passage with a length between afirst opening and a second opening, wherein the first opening of thesecond passage is located at an external surface of the downstream faceof the seal pack carrier, and the second opening of the second passagecommunicates with the low-pressure plenum, wherein high-pressure isprovided to the high-pressure plenum through the first passage and aregion of the gas turbine engine which, during operation of the gasturbine engine, contains fluid at higher pressure than fluid in thedownstream region adjacent the compliant seal assembly such that a forceis applied to the seal pack in the upstream direction, and low-pressureis provided so that the low-pressure plenum is vented to the downstreamregion through the second passage.